Overhead gantry cranes are generally known for lifting heavy items. For example, such cranes are used for handling large products or containers and transporting them to and from storage or loading containers on ships, trains, trucks, etc. These cranes are commonly used in the construction industry as well, handling large construction materials, such as beams, blocks, concrete barriers, pipeline sections, prefabricated components, etc.
Known cranes of a type referred to herein usually include two parallel horizontal beams which are supported in an elevated fashion by a frame. Each of these horizontal beams is equipped with a lifting hoist trolley which is actively traversible along the horizontal beam in a motorized manner by a positive drive system. Each trolley is a part of a hoist having a separate lifting cable system operable to lift a load, usually with a lifting block suspended by the cable from the trolley. For moving the entire crane, the frame is mounted on drivable and steerable wheels so that an operator can drive the crane over a site to lift or deposit a load at a desired location.
A specific example of a crane application is in the prestressed concrete industry, where gantry cranes are used for handling slabs or panels of formed concrete. These concrete panels are conventionally cast in a horizontal position, but often must be stored and/or transported in a vertical orientation. Therefore, after casting, the concrete panels must not only be lifted, but also must be pivoted from horizontal to vertical orientations for storage or transport. Additionally, at a construction site these concrete panels must be removed from vertical cargo or storage positions and be pivotally positioned horizontally for installation, e.g., as flooring sections.
Because concrete panels can weigh several tons apiece, and because they are breakable, these handling situations require careful crane operation to pivot the load. Load pivoting has conventionally been accomplished by a crane equipped with two positively-driven hoist trolleys on each of the two horizontal beams, for a total of four positively-driven hoist trolleys per crane. On each beam, one of the hoists is operated to lift one side of the load, while the second hoist is operated to lift the other side of the load. Then, by raising or lowering only one of the hoists, that side of the load is accordingly raised or lowered, "pivoting" the load.
Because the panel may have a substantial width dimension, the two hoist trolleys per beam must be drivably positioned at generally that same panel-width dimension apart from each other, vertically above the respective sides of the horizontal panel. To pivot the panel to a substantially vertical orientation, the two hoist trolleys on the beam must be positively driven closer to each other during the pivoting operation (while one of the hoists lifts or lowers its side of the panel). Likewise, the two hoist trolleys must be positively driven apart from each other while pivoting a panel from a vertical to a horizontal position. With such a crane having dual positively-driven hoists, pivoting a wide load has proven to be tedious, as it requires an operator to actively control the lateral movement of one or both positively-driven hoists while simultaneously controlling the lifting movement. It is desirable to reduce the number of control functions performed by a human operator in order to increase productivity and accuracy.
It has also been known to use two entire overhead crane vehicles to perform a lifting and pivoting of a single load. One hoist from each of the two cranes lifts a respective end of the item, and the coordinated raising or lowering of the two independent hoists is operable to pivot the load. During the pivot, the two cranes can be moved closer or farther from each other in order to obtain a correct pivot angle.
The use of two cranes is impractical for several reasons. Where two cranes are used, additional costs are required for the purchase of a second crane and employment of a second crane operator. Also, it is difficult for two crane operators to coordinate separate cranes to carry out the calculated task of lifting and pivoting a multi-ton object.
Thus, lifting and pivoting with conventional cranes has required expensive components, either in the form of a second crane or in equipping a crane with dual positively-driven hoists.
It is further noted that cranes are known wherein a single trolley carries two hoists, particularly for lifting and dumping a bucket-like vessel. For example, such devices are disclosed in U.S. Pat. Nos. 3,297,170, 3,854,592, 4,144,974, and 4,360,304. These trolleys might be suitable for pivoting a bucket or some other load which does not have a substantial width dimension, however, they are not suited for pivoting wide objects such as panels. In particular, the hoists are fixed closely together in these devices. If used to lift and pivot a load suspended at widely-separated lifting points, the cables would be lifting at non-vertical angles, thereby subjecting them to amplified tensile stress due to both vertical and horizontal force components. Other structural elements of the hoists would also be subjected to such amplified stress, such as the trolley sheaves over which the cables run, the hoist winches, etc. Therefore, it is desirable to provide a crane capable of maintaining a substantially vertical cable orientation during the lifting and pivoting of a load.